1. Field of the Invention
The present invention relates generally to image sensing apparatus, and more specifically to a technique for cancelling an amplitude difference between signals shifted along respective shift registers of an image sensor. The present invention is useful for application to facsimile machines and earth observation satellites.
2. Description of the Related Art
Multi-element image sensing apparatus used in applications such as facsimile and earth surveillance include a calibration system for periodically checking the functions of the apparatus in addition to circuitry for sensing images. In these applications, a video signal is generated when the object of interest has a sufficient level of luminance for viewing, and a calibration is performed at other times. More specifically, the image sensing apparatus comprises a linear array of image sensing elements and two shift registers one of which is connected to the odd-numbered elements of the array and the other is connected to the even-numbered elements of the array. Due to manufacturing tolerances, the operating characteristics of the shift registers are not necessary the same. During low luminance periods, a calibration pulse sequence of known amplitude is shifted along the shift registers and the sequences delivered from the shift registers are matched against each other to detect an amplitude difference. In response to the amplitude difference, calibration is effected on the image signals subsequently output from the shift registers.
Since image sensing and calibration are effected during different periods of time, however, the operating characteristics of the shift registers at the time of calibration may differ from those of the shift registers during image sensing due to constantly varying environmental conditions such as temperature and humidity. Therefore, the result of a calibration provided at one time is not ideal for an error that occurs at another time.